Scientists Say Frequent Extreme Weather Events Not 'Coincidental'

3/31/2012 1:21 PM ET

Extreme weather events, such as heat waves and tropical cyclones, are usually rare and can usually be attributed to causes local to the site of occurrence. However, scientists at the Potsdam Institute of Climate Science are questioning whether the recent rise in frequency of extreme weather incidences is truly "coincidental."

In a paper titled "A Decade of Weather Extremes," published online in a recent issue of Nature Climate Change, scientists argue that there may be a connection between these events and global warming.

As lead author Dim Coumou puts it: "Global warming can generally not be proven to cause individual extreme events - but in the sum of events, the link to climate change becomes clear."

This conclusion follows a fresh analysis of published studies of extreme events - 14 of which were recorded in the U.S. alone in 2011. There is "strong" evidence, as the authors put it, relating such events as heatwaves and "precipitation extremes," such as heavy snowfall or rainfall, to global warming. However, this is not the case with storms.

The latest review uses, in part, knowledge about extreme events derived from basic physics, which points to a warmer atmosphere as a possible cause for the extreme events. This acts through such effects as holding more moisture. Further, statistical analysis reveals trends in temperature and precipitation data.

The use of detailed computer simulations helped connect these dots, confirming a nexus between warming, and the temperature and precipitation trends. This "dependency" however, can be complex in certain cases, the scientists found, and additionally, not all historic storm data is precise and reliable.

One phenomenon that poses such questions is the spike in North Atlantic tropical storm intensity observed between the years 1980 and 2005. While warmer oceans could be one cause, a cooler upper atmosphere can also have a similar impact. It is understood that an increase in ocean temperature can affect the intensity of a tropical storm, but this is not known to play a role in the frequency of such storms.

"These are complex processes that we are investigating further." noted co-author Stefan Rahmstorf, adding "But now these processes unfold against the background of climatic warming. That can turn an extreme event into a record-breaking event."